Method for treating steel works dust by wet process

ABSTRACT

A method for treating steel works dust in order to recuperate elements capable of being upgraded. The method comprises attrition in water followed by hydraulic grading of the resulting load. The method is characterized in that is further comprises: a washing step to separate the water soluble saline fractions of the insoluble oxides; hot treatment to eliminate metals in the form of free oxides such as zinc and lead; treatment by heating at a temperature ranging between 240 and 800° C.; treatment with sulphuric acid having a concentration between 5 and 8%.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to the technical field of the treatment of dustfrom steelworks, notably electric steelworks.

BRIEF SUMMARY OF THE INVENTION

According to a first general object, the invention discloses a methodand an installation for treating said dust allowing elimination of theheavy metals it contains.

According to a second general object, the invention discloses a methodand an installation for treating said dust allowing maximum economicrecovery of the most abundant fraction of this dust.

The term heavy metal, which has come into use in many legislations,relates to metals which are generally:

non-degradable over time;

toxic to living systems at very low concentrations;

inclined to accumulate in living organisms and to become concentrated inthe course of transfers of materials in food chains.

Steelworks dust, for example issuing from remelting in the electricfurnace of scrap iron for the production of steel, contains such heavymetals such as zinc, cadmium and lead, for example.

Dust production is between 15 and 22 kg per tonne of liquid steel.

As a guide, steel production, in Europe, was of the order of over500,000 tonnes per year at the beginning of the 1990s.

DESCRIPTION OF THE PRIOR ART

Consequently, there is a considerable need for effective treatment ofthis dust, for the sake of protecting the environment notably.

A number of methods have been envisaged for the treatment of this dust,the methods varying depending on the composition of said dust.

Two main classes of dust are distinguished, depending on the nature ofthe steels produced, carbon steels and stainless alloyed steels.

In the case of carbon steels, zinc (17 to 29%) is present in the dust intwo forms: zinc oxide, ZnO, and zinc ferrite, ZnFe₂O₄, while the lead (4to 5%) is in oxide form (PbO) The cadmium contents are lower, of theorder of around 800 ppm.

Unlike zinc oxide, zinc ferrites are not easily soluble duringhydrometallurgical treatments for releasing the zinc.

For alloyed and stainless steels, the quantity of dust generated inEurope was around 50,000 tonnes per year during the 1990s.

Various procedures and processes, many still at the experimentallaboratory stage, have been devised for the treatment of dust fromsteelworks, notably electric steelworks.

The BUS (BERZELIUS UMWELT) procedure proposes a treatment of dust fromsteelworks for carbon steels combining two processes: the WAELZ processand the ISP (IMPERIAL SMELTING PROCESS). This procedure uses aconventional pyrometallurgical process and condensation of the zincvapours by means of liquid lead spray condensers.

The FLAME REACTOR process, used for the treatment of dust from alloyedand stainless steels, consists essentially of vertical flamesmelting/reduction cyclone separation. The waste products, rich in zincand lead, are converted into slag to be resmelted and into oxide to berecovered.

The PLASMADUST process has been used since the 1980s for the treatmentof electric steelworks dust rich in zinc and lead. The treatment furnaceis of the tank furnace type, whose energy is provided by blown arcplasma torches. The energy of the plasma is used to heat the reagents,and to provide the heat necessary for the endothermic reductionreactions. The plasma-producing gas is introduced into the furnace bywater-cooled copper nozzles. The powdery charge composed of dust, fluxand coal is injected into the nozzles and mixed with theplasma-producing gas. The fumes charged with metallic vapours leave thefurnace at a temperature of around 1150° C.

When the PLASMADUST process is used for dust from alloyed and stainlesssteels, the zinc content in the fumes is generally too small forcondensation of the zinc to be efficient, the zinc in this case beingretrieved from the sludge issuing from the washers.

The PLASMINOX process uses a treatment by means of hot cathode plasmatorches operating by transferred arc at D.C. and installed vertically onthe furnace. In the furnace, the metals present—chromium, nickel,molybdenum and iron—are separated from the dust by melting and theferroalloy which results therefrom is poured into moulds for subsequentrecharging at the dust-producing steelworks. The scorias obtained are,in accordance with the standards, inert. The fumes escaping from thefurnace are cooled and have their dust removed. The secondary dustobtained contains volatile metals Z_(n), P_(b) which are in the form ofoxides and are not recovered.

The modified ZINCEX process produces zinc by hydrometallurgical means,this process having the following successive steps:

atmospheric leaching of the dust by a dilute sulphuric acid solution;

purification of the leaching solution by precipitation of the iron andaluminium;

selective extraction of the zinc by D2EHPA;

stripping of the zinc from the organic phase by the highly acidelectrolytic solution;

electrodeposition of the zinc on to aluminium cathodes.

The ZINCEX process is relatively complex in its implementation since itrequires organic solvents combined with sulphuric acid. Furthermore,this process requires large quantities of wash water to avoid theformation of SO₂ from the oxides which are recycled in the furnace. ThisZINCEX process is not easily exportable, owing to the above-mentionedproblems and since it necessitates too heavy investments.

The GLASSIFICATION process comprises a step of mixing the steelworksdust with other steelwork waste products and constituents of glass. Themixture is melted in a submerged arc furnace. The metals vaporizedduring the melting condense in the upper part of the furnace beforebeing trapped in the melt. The glass obtained can be put in the form ofgranules for the manufacture of glass tiles or grit.

In total, three main ways of recycling have been envisaged for recyclingdust from electric steelworks.

A first way consists of performing reductions at various temperatures inaccordance with the processes for vaporizing the heavy metals andre-injecting the ferrous fraction into the steel production furnaces:

the document EP 336 923 proposes treating steelworks dust in an ironsmelting furnace after conversion into pellets;

the document EP 441 052 proposes a thermal treatment between 1200 and1700° C. by the addition of reducing agents;

the document WO 91/12 210 describes a method of treating steelworks dustin an iron reduction furnace, the heavy metals being retrieved bycondensation from the hot gases;

the document EP 453 151 describes a method of treating dust in the formof pellets by an agent selectively reducing iron oxide;

the document FR 2 666 592 describes a device for extracting volatilemetals acting by oxidation;

the document WO 93/69 619 describes an arc furnace specially designedfor the reduction of dust containing oxides;

the document EP 551 992 describes a method of retrieving recoverablemetals from steelworks dust by reduction and vaporization of therecoverable metals;

the document FR 2 373 612 describes a method of extracting zinccontained in steelworks dust, by selective oxidation under heat.

A second way consists of directing the dust towards a use where it wouldbe “made inert” by various means with a view to use as construction orfiller materials:

the document EP 402 746 proposes recycling by incorporation in a clayfor a mine packing material;

the document WO 91/12 210 proposes the use of steelworks dust for thetreatment of sewage with the addition of a flocculating agent;

the document FR 2 689 881 describes a method of manufacturing brickswhose properties are improved by the addition of electric steelworksdust first calcined at the vaporization temperature of heavy metalscontained in said dust;

the document FR 2 700 161 describes a mixture for a road surfacingcomprising 2 to 6% by weight of electric steelworks filter dust.

Pyrometallurgical processes, with or without electricity, allow therecovery of oxides of zinc, of iron or nickel chromium alloy or ofliquid zinc most often by condensation.

A third way, hydrometallurgical, has also been envisaged.

Various media such as sulphuric acid, hydrochloric acid, soda andammonia have been tried out.

The document FR 2 716 895 describes a method of treating electricsteelworks dust with or without prior reduction, or electrolyticgalvanizing dust. Said method comprises an ammoniacal attack by anammoniacal buffer solution, then a desorption of the ammonia by air flowcleaning.

Direct attacks in a sulphuric or hydrochloric medium are noteconomically viable, since the volumes in terms of iron hydroxide orsalts involved are enormous.

These residues are moreover still contaminated by metallic impuritiessuch as lead and cadmium.

Alkaline leaching has been proposed.

The document FR 2 535 736 describes a method of treating dust containingzinc, issuing from electric steelwork furnaces, in which the dust issubjected to a basic leaching. The basic rejects resulting from thedifferent washing phases and containing zinc are neutralized by means ofthe acid rejects coming from galvanization operations and alsocontaining zinc.

The document FR 2 501 141 describes a basic leaching of zinc-bearingdust by means of ammonium chloride, the powder obtained beingagglomerated and introduced into a smelting furnace.

By the leaching technique, only the elements in the form of free oxides(mainly zinc and lead) are dissolved. The mixed oxides of non-ferrousmetals/ferrous oxides are not affected. Their management is thereforeproblematic, including re-injection into the steelworks furnace whichleads, in the end, to a problem of re-concentration of impurities.

The methods combining liquid ammonia with ammonium salts (notably theENGITEC process described in European patent application EP-93 00018.5)have drawbacks in the handling of chlorides for the retrieval ofminority elements (zinc, lead and cadmium) and the recovery of ironoxides. The high chloride content prevents recycling of these phases inthe furnaces owing to the problems related to chlorine. Their washingwould require considerable volumes of water.

It can therefore be thought that this type of method has a maximumefficiency between 25 and 30%, proportional to the concentration in thenon-ferrous phase. This method therefore generates a waste product whosemanagement remains as problematic as that of the untreated dust.

The document FR 2 737 503 describes a method of preparing mineralpigments, and the mineral pigments thus obtained, and an installationfor the implementation of such a method. This document, coming from theapplicant, describes a method having the following steps:

separation of the dust into two fractions, one fraction comprisingmagnetic elements and one fraction comprising non-magnetic elements;

basic leaching of the non-magnetic fraction in order to dissolve thezinc fraction which would not be bound up in spinel form, and thesilica, lead, and a fraction of the manganese;

rinsing until neutralized and separation of the charge thus obtained;

calcination at a temperature between 450 and 650° C. of the charge thusobtained;

treatment of the calcined charge with sulphuric acid in the presence ofa catalyst for solubilizing the iron oxides formed during thecalcination step and the heavy metals other than zinc;

collection of the mineral pigments;

use of the solutions issuing from the rinsing and the sulphuric acidtreatment for precipitating other pigments.

The present invention proposes a method of neutralizing the free heavymetals contained in the dust from steelworks, notably electricsteelworks, said method also allowing the recovery of the most abundantfraction of said dust.

The method according to the invention makes it possible to treatindiscriminately the so-called acid dust and the so-called basic dust.

The method according to the invention makes it possible togeographically dissociate pre-treatment of the dust at the collectionsite and chemical treatment and recovery from said dust.

The method according to the invention is particularly adapted to thetreatment of dust from carbon steels and tool steels.

The invention relates, according to a first aspect, to a method oftreating steelworks dust with a view to retrieval of the recoverableelements, said method comprising an attrition in water followed by awater grading of the charge thus obtained, said method beingcharacterised in that it also comprises:

washing of the charge collected at the overflow issuing from the watergrading, said charge being charged with dissolved heavy metals andsalts, with a view to separating the water-soluble saline fractions fromthe insoluble oxides,

treatment under heat of the whole of the charge, washed in the previouswashing step without prior magnetic separation, with a view toeliminating the metals in the form of free oxides such as zinc and lead,

treatment by thermal means of the charge obtained after treatment underheat at a temperature between 240 and 800° C.

The method also has the following aspects, possibly combined.

A step of treatment with sulphuric acid with a concentration of between5 and 8% is carried out after the treatment by thermal means.

The treatment under heat performed after the washing step is, accordingto a first variant, carried out in an acid medium. Said acid medium is,in one embodiment, a sulphuric acid solution at a concentration ofbetween 8 and 18% and a temperature of between 40 and 95° C.

The treatment under heat is followed by an oxidation and then aseparation of the iron in the form of an insoluble oxide precipitate.

The zinc is separated by means of an acid electrolysis.

The treatment under heat performed after the washing step is, accordingto a second variant, carried out in an alkaline medium.

According to one embodiment, the washing comprises treatment with asulphur derivative chosen from amongst the group of heterocycliccompounds such as trimethyl triazine or 2,5 dimercapto thiadiazole.

The alkaline treatment is performed in a concentrated soda solutionunder heat in the presence of an oxidizing agent.

The concentration of the soda solution is between 240 and 400 g/l.

The temperature of the alkaline treatment is between 50 and 110° C.

The oxidizing agent is chosen from amongst the compounds which areoxidizing in an alkaline medium such as hydrogen peroxide.

The hydrogen peroxide concentration is between 10 and 40%, and moreparticularly between 30 and 35%. The lead contained in the alkalinesolution is eliminated by means of hydrogen peroxide with aconcentration equal to at least 35% by weight.

The treatment by thermal means is performed between 400 and 650° C.

The treatment with sulphuric acid is carried out in the presence of aninjection of air in order to oxidize the divalent iron into trivalentiron.

The treatment with sulphuric acid is carried out under heat at atemperature between 40 and 80° C.

The solution issuing from the acid attack is adjusted to a pH between5.4 and 10.

The solution issuing from the acid attack is adjusted to a temperaturebetween 25 and 80° C.

The solution issuing from the acid treatment is purified by means ofiron or zinc powders.

The method also comprises a two-stage electrolysis of the solutionissuing from the alkaline treatment.

Said first electrolysis uses a cathode of graphite in grains containedin a basket in contact with a titanium grid.

The basket is made of polypropylene.

The anode is made of titanium containing ruthenium or iridium.

The second electrolysis comprises an anodic re-dissolution of thegraphite cathodes in a sodium sulphate solution and a cathodicre-deposition of the zinc on an iron or steel electrode.

The second electrolysis is carried out in an electrolyser whose pH isadjusted to a value close to 5.

Other objects and advantages of the invention will emerge in the courseof the following description of embodiments, said description beinggiven with reference to the accompanying figures in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting the steps of the method,according to one embodiment, the numerical references contained in acircle corresponding to the intermediate products obtained during themethod, the final products being notably the products P1, P2, P3, P4, P5and S;

FIG. 2 is a schematic diagram showing a configuration with a number ofsub-units A for pre-treatment of steelworks dust, associated with asingle sub-unit for physico-chemical treatment B of the fines issuingfrom the pre-treatment;

FIG. 3 is a graph depicting the variations in zinc concentration overtime, during electrolysis of a solution at 80A and 3V. The zincconcentration C, in grams per litre of solution, is given along they-axis, the time t in hours being along the x-axis.

DETAILED DESCRIPTION OF THE INVENTORS

A few general technical data are first presented before describing theinvention in different embodiments.

Steelworks dust results from at least two reactions.

The most volatile metals pass into the vapour phase at the furnaceoperating temperature. Under the effect of air suction, these volatilemetals are then oxidized and cooled and find themselves either in theform of free oxides, or in the form of structures mixed with ironoxides.

Above the molten melt, fine particles of iron are dispersed in thevapour phase and carried along by the air suction. In this air current,they are cooled and, under the action of the oxygen in the air,converted into higher oxides.

Interaction between these oxides and the heavy metals can lead to theformation of spinel type compounds MFe₂O₄ (M=zinc, nickel, manganese orcadmium). If M is iron, the phase is composed of magnetite.

Electric steelworks dust therefore contains variable quantities of iron,zinc, calcium and silicon type majority elements in the form of simpleor mixed oxides and minority elements such as copper, manganese,chromium, cadmium, lead and the chlorides.

The dust is considered toxic because of graining out of the heavy metalsby leaching and therefore comes under Class 1 discharges.

On this subject, the regulations in France provide that, from 2002, onlywaste products said and confirmed to be final will be accepted in Class1 discharges.

Electric steelworks dust has very broad granulometric distributionssince it contains fractions between 0.1 and 150 microns.

Moreover, this dust has a strong tendency to agglomerate.

This property rules out any possibility of dry screening, including thatassisted by ultrasound.

Granulometric study of electric steelworks dust shows that the chemicalcomposition of the fractions depends greatly on the granulometry.

Thus, the largest fractions are the richest in metallic iron.

As the dust also results from physical agglomeration of small-sizedparticles around a larger central grain, only an attrition in watermakes it possible to break up the agglomerates. This advantageouslyfacilitates the separation of the charge into two fractions (larger orsmaller than 40 microns for example).

The attrition operation assumes great importance since it makes itpossible:

to accelerate the dissolution of the soluble salts;

to reveal the metallic fractions which are masked by the oxides absorbedon the surface.

The steelworks dust treatment method according to the inventioncomprises first an attrition in water of untreated dust 1.

The solid/liquid ratio must be judiciously chosen in order to putsuitable friction between the solid particles while providing asufficient volume to ensure dissolution of the soluble fractions.

A solid/liquid ratio between 2.3 and 2.5 is satisfactory in thisrespect.

After the attrition operation 2, the charge 3 is diluted (operation 4)sufficiently to complete the dissolution of the salts, put the very fineparticles into suspension, and provide a pulp density 5 adapted to awater grading 6.

The water grading 6 is advantageously performed in hydrocyclones withcut-points adapted to the granulometry of the incoming dust.

The hydrocyclones operate in continuous mode and are essentiallyconstituted by a vertical cylinder where the separation takes place,provided at its base with a cone for retrieving the solid, closed by ahorizontal surface at its upper part.

The pulp 5 is introduced tangentially into the upper part of thecylinder at high speed.

The charge collected at the underflow 7 of the cyclones is rich incarbon, iron and magnetite. This charge is decanted, dried (operationsreferenced under the number 8) and the product P1 obtained can bere-injected into the steel production furnaces 9.

The charge collected at the overflow 10 is charged with dissolved heavymetals and salts.

According to one particular embodiment, the charge is then treated at 11with a sulphur derivative chosen from amongst the heterocyclic compoundssuch as trimethyl triazine (TMT) or 2,5 dimercapto thiadiazole (DMT) inorder to eliminate all the heavy metals in solution without additionalmineralization.

The saline solution obtained 12 is then subjected to a concentration at13 in order to retrieve the salts S and recycle the water 14 thuspurified.

This concentration step 13 is carried out by evaporation,crystallization or concentration by membrane.

According to a first variant, the charge 15 issuing from theconcentration step is subjected to an acid treatment.

Thus, for example, 50 kg of dust issuing from the washing is attacked bya 15% sulphuric acid solution at a temperature of 80° C. for 3 hours.

After filtration and separation of the insoluble residue, the pH of thesolution is brought back to a value of 3.8 by means of iron powder, andthen this solution is oxidized by means of hydrogen peroxide in astoichiometrical ratio with respect to the iron. An orange-redprecipitate appears.

The suspension obtained is heated at 90° C. for one hour, then filteredand the separated solid is backwashed in a filter press.

The solution obtained by filtration contains an iron concentration lowerthan 1 mg/l and around 18 g/l of zinc. It is subjected to anelectrolysis in an acid medium leading to the production of solid zinc.

According to a second variant, the charge 15 issuing from theconcentration step 13 is treated at 16 with a concentrated soda solutionunder heat in the presence of a specific oxidizing agent.

This oxidizing agent can be chosen from amongst the compounds which areoxidizing in an alkaline medium.

Hydrogen peroxide or ozone can advantageously be chosen with a view tonon-contamination of the medium.

Water and oxygen are the only products obtained after reaction of H₂O₂.

This concentrated soda treatment step 16 makes it possible to dissolve:

the fraction of the zinc which would not be bound up in spinel form;

the silica;

the lead.

After this first chemical treatment, the solid charge obtained 17 isrinsed until neutralized and separated at 18 by the most appropriateseparation means.

This step 18 can be performed by means of a membrane filter press with abackwashing device.

The solid obtained 19 is then calcined at 20 at a temperature between400 and 650° C. in a fluidized bed furnace capable of accepting sludgeat the input.

If the calcination temperature is not within the stated range,undesirable crystallographic structures may form.

A calcined charge 21 is thus obtained, this charge 21 coming out inpowder form, this avoiding an expensive grinding. This powder P2 mayfind applications in the pigment field.

The calcined charge 21 can also be treated at 22 with moderatelyconcentrated sulphuric acid in the presence of an injection of air inorder to oxidize the divalent iron into trivalent iron.

This operation can be performed under heat.

The alkaline attack solution 23 issuing from the soda treatment 16 ischarged with zinc and lead.

The solution 23 is purified at 24 by the addition of zinc powder 25 inorder to carry out cementation of the lead. This zinc powder can be thatobtained by the method of the invention.

Cementation here means a deposition by reduction in which a noble metalbody, oxidized and dissolved, is reduced by another metal body added tothe solution in powder form. Cementation is a known process for metalssuch as As, Cd, Co, Cu, Pb, Ni, Sb and Sn.

In the case of untreated dust 1 rich in lead, according to oneembodiment of the invention, the lead can be eliminated by oxidation.

Thus, by way of an illustrative example: to one litre of alkalinesolution 23 containing 11 g/l dissolved lead in the form of plumbate,there is added 60 ml of hydrogen peroxide at 35% by weight in successivefractions of 20 ml.

The plumbate concentration falls from 11 g/l to 5 g/l and then to 0.8g/l.

Through this variant, it is possible:

to obtain a relatively pure lead oxide with a view to better recovery;

to save on zinc powder which is not used in this variant to carry outcementation of the lead.

The purified solution obtained 26 is slightly more concentrated in zincis subjected to an electrolysis at 27.

It is known in the prior art that electrolysis of alkaline solutions ofzinc (dissolved in the form of zincate) leads to the formation of highlyreactive zinc powder.

This is because these powders oxidize spontaneously in air and arepyrophoric.

The handling of these powders is therefore very tricky. Full of soda,these powders must be abundantly rinsed and passivated.

The only reagent giving convincing passivation results is sodiumdichromate. Thus, the alkaline electrolysis of zincates forces theintroduction of chromium VI, a dangerous pollutant.

In order to avoid the drawbacks of the formation and handling of zincpowder, the electrolysis 27 is performed, in the invention, by a cellwith two compartments.

The cathode compartment comprises a titanium electrode which providesthe electrical contact with graphite balls on which the zinc will bedeposited.

The cathode of graphite in grains is contained in a basket in contactwith a titanium grid.

These balls are placed in a polymer basket with a mesh adapted to thegranulometry of the graphite.

In one embodiment, the polymer used for producing the basket ispolypropylene.

The anode is made of titanium containing ruthenium or iridium.

After the electrolysis step 27, the soda-enriched solution 28 isreturned to the top of the process to be used for attacking the freezinc.

The cathode compartment assembly (titanium+graphite+deposited zinc) isimmersed in a so-called “regeneration” electrolyser for the zinccontaining demineralized water.

The pH of the regeneration electrolyser is adjusted to a value close to5 in order to avoid the cathodic mass charged with soda causing a risein pH.

The cathode is made of aluminium or iron.

The assembly comprising the titanium+graphite+initially deposited zinccompartment is polarized anodically so as to cause dissolution of thezinc.

The electrolysis 27 then leads to a deposit of zinc 29 on the metalplates of iron or aluminium, by cathodic redeposition of the zinc onthese electrode-forming plates, made of iron or steel.

Some of this zinc 29 can be used in the cementation step 24.

The zinc obtained 29 forms a directly marketable product P3.

After alkaline treatment 16 and thermal treatment at 20, the charge 30is subjected to a selective acid attack at 31 in order to dissolve theiron oxide without affecting the zinc ferrite phases.

The product obtained 32 is then washed and dried at 33.

The final product 34 is composed essentially of zinc ferrite whose shadeof colour varies according to the degree of reduction and the zinc/ironratio. This product 34, P4, can be used in the pigment industry.

The solution 35 obtained during the acid attack 31 is reserved for theremainder of the process.

This solution 35 will be used to produce iron oxyhydroxides byneutralization with soda using the alkaline charge wash waters.

In one embodiment, the underflow 7 issuing from the water grading 6 issubjected to an acid attack 36 by combining the solution 35 issuing fromthe alkaline charge attack and an addition of new acid in order toprovide the dissolution of the ferrous compounds.

The controlled neutralization 37 of this solution leads to theprecipitation of an iron oxyhydroxide 38 of FeOOH type.

The product 38 washed and dried at 39 has, in its final form P5,different shades of colour depending on the thermal treatmenttemperature.

This product P5 can be used as a pigment for applications in theconstruction industry for example.

The method and the installation just described define at least twodistinct sub-units:

a first sub-unit A in which the steps 2 (attrition), 4 (dilution) and 6(water grading) are carried out, leading to an overflow 10 and anunderflow 7;

a second sub-unit B for physico-chemical treatment of the overflow 10.

The sub-units A and B can correspond to two installations geographicallydistant from one another.

In other words, the pre-treatment of the dust, for example on itscollection site, said pre-treatment corresponding to the underflow, A,can be dissociated from a sub-unit B for physico-chemical treatment ofthe fines of the overflow 10.

As depicted in FIG. 2, a single sub-unit B can provide the treatment ofthe overflow fines issuing from a plurality of pre-treatment sub-unitsA.

Physico-chemical treatment and recovery of the overflow fines 10 is thuscentralized and carried out on very large volumes, hence increasedprofitability.

As required, the waters 14 issuing from the concentration step 13 can beused for the attrition 2 and/or the dilution 4.

Similarly, in one embodiment, the underflow 7 can be involved in thedissolution of the ferrous compounds of the solution 35.

The sub-units B and A can then be geographically close.

EXAMPLE

17 kg of untreated dust 1 are subjected to an attrition 2 for 15 minutesin 7 litres of water.

After attrition 2, the charge 3 is diluted at 4 in 120 litres of waterthen subjected to a water grading 6 through a cyclone performing a cutat 5 microns.

Evaluation of the granulometric distribution shows two clusters centred:

on 5 microns for the finest fraction (overflow 10). Analysis by scanningelectron microscopy of this phase shows a very fine homogeneous powdermorphology. This phase 10 is very rich in zinc, the concentration ofwhich may reach 30%.

on 100 microns for the residue 7 which constitutes 14% of the incomingcharge 1. Analysis by scanning electron microscopy of this phase 7 showsthe presence of balls of regular shape whose iron content isconsiderable (80%). This phase 7 contains only 6% zinc.

After separation of solid and liquid, the water is subjected to adepollution treatment.

100 litres of water 10 are treated by means of 4.5 g of 2,5 dimercaptothiadiazole.

After agitation for one hour, a flocculent is added and the solution isfiltered through a filter press.

Analysis shows that the elements such as zinc, lead, mercury and cadmiumare at a concentration lower than 0.05 mg/l.

The clear solution is subjected to a reverse osmosis treatment in orderto obtain a brine at 90 g/l and a water 14 of industrial qualityre-usable at steps 2 and 4 of the process notably.

200 kg of the charge 15 are subjected to a basic leaching 16 by means ofsoda at 30% by weight, for 30 minutes at a temperature of 90° C.

After filtration, the solid fraction 17 is washed until neutralized thendried and calcined at 20 at a temperature of 400° C. for 4 hours.

The product P2, 21 obtained already finds applications for example as anadditional charge in pigments based on iron oxides.

The solid charge cleared of soluble salts and heavy metals is treatedwith sulphuric acid with a concentration of between 5 and 18% for 2hours at 60° C. Throughout the duration of the reaction, air or oxygenis injected at the base of the attack reactor.

The solid 34, P4, obtained meets the characteristics of the nomenclatureof pigments based on iron oxides.

The solution 35 issuing from the acid treatment 31 can be treatedaccording to the method described in the patent FR 95 09 548, throughneutralization 37, at 60° C. to a pH of 5.4 while maintaining a bubblingthrough of air at 600 l/h.

The precipitate obtained 38 is washed and dried at 250° C. in acirculating air drier.

What is claimed is:
 1. A method of treating steelworks dust with a viewto retrieval of the recoverable elements, said method comprising anattrition in water (2) to obtain a charge followed by a water grading(6) of the charge (5) thus obtained wherein an overflow (10) issues fromsaid water grading, said method being characterized in that it alsocomprises: washing (11) of a charge collected at the overflow (10)issuing from the water grading (6), said charge being charged withdissolved heavy metals and salts, with a view to separatingwater-soluble saline fractions from insoluble oxides, treating underheat (16) of the whole of the charge, washed in the previous washingstep (11) without prior magnetic separation, with a view to eliminatingthe metals in the form of free oxides, treating by thermal means (20) ofa charge (17) obtained after treatment under heat (16) at a temperaturebetween 240 and 800° C.
 2. A method according to claim 1, characterizedin that a step of treatment with sulfuric acid (22) with a concentrationof between 5 and 8% by weight is carried out after the treatment bythermal means (20).
 3. A method according to claim 1, characterized inthat the treatment under heat (16) performed after the washing step (11)is carried out in an acid medium.
 4. A method according to claim 3,characterized in that the treatment under heat in an acid medium isperformed in a sulfuric acid solution at a concentration of between 8and 18% by weight and a temperature of between 40 and 95° C.
 5. A methodaccording to claim 3, characterized in that the treatment under heatstep (16) is followed by an oxidation and then a separation of iron inthe form of an insoluble oxide precipitate.
 6. A method according toclaim 1, characterized in that a metal separated in the form of a freeoxide by means of an acid electrolysis is zinc.
 7. A method according toclaim 1, characterized in that the treatment under heat (16) is analkaline treatment performed in a concentrated soda solution under heatin the presence of an oxidizing agent, the concentration of the sodasolution being between 240 and 400 g/l, and the temperature of thealkaline treatment (16) being between 50 and 110° C.
 8. A methodaccording to claim 7, characterized in that the oxidizing agent ischosen from amongst compounds which are oxidizing in an alkaline medium.9. A method according to claim 8, characterized in that the alkalinemedium is hydrogen peroxide having a concentration between 10 and 40% byweight.
 10. A method according to claim 8, characterized in that thehydrogen peroxide concentration is between 30 and 35% by weight, a metalcontained in the alkaline solution (23) is lead, the lead beingeliminated by hydrogen peroxide with a concentration equal to at least35% by weight.
 11. A method according to claim 1, characterized in thatthe treatment by thermal means (20) is performed between 400 and 650° C.12. A method according to claim 5, characterized in that the treatmentunder heat step (16) is a treatment with sulfuric acid (22) which iscarried out under heat at a temperature between 40 and 80° C.
 13. Amethod according to claim 12, characterized in that the solution issuingfrom the sulfuric acid treatment is adjusted to a pH between 5.4 and 10.14. A method according to claim 12, characterized in that the solutionissuing from the sulfuric acid treatment is adjusted to a temperaturebetween 25 and 80° C.
 15. A method according to claim 12, characterizedin that the solution issuing from the sulfuric acid treatment ispurified by means of iron or zinc powders.
 16. A method according toclaim 7, characterized in that it also comprises a two-stageelectrolysis (27) of a concentrated soda solution (26) issuing from thealkaline treatment (16).
 17. A method according to claim 16,characterized in that the metal is zinc, and that the first electrolysisstage consists of a reduction of zinc in a concentrated basic medium.18. A method according to claim 17, characterized in that said firstelectrolysis stage uses a cathode of graphite in grains contained in abasket in contact with a titanium grid.
 19. A method according to claim18, characterized in that the basket is made of polypropylene.
 20. Amethod according to claim 16, characterized in that the electrolysisincludes the use of an anode made of titanium containing ruthenium oriridium.
 21. A method according to claim 18, characterized in that thesecond electrolysis stage comprises an anodic re-dissolution of thegraphite cathode in a sodium sulphate solution and a cathodicre-deposition of the zinc on an iron or steel electrode.
 22. A methodaccording to claim 21, characterized in that the second electrolysis iscarried out in an electrolyser whose pH is adjusted to a value close to5.